This invention relates to a novel method of producing microcapsules. More specifically, it relates to the preparation of microcapsules having walls of low permeability and high mechanical and thermal stability.
Microcapsules containing solid, liquid gaseous materials have been used commercially in various applications for several years. One of the most common uses of such microcapsules has been in pressure sensitive copy systems wherein a chromogenic material dissolved in an oily solvent is encapsulated and coated onto a transfer substrate. With the application of localized pressure such as from a stylus, typewriter, pen, or other implement, the microcapsules rupture and the chromogen solution is transferred to an underlying sheet having an absorbent coating of a Lewis acid material which reacts with the chromogen released from the ruptured microcapsules to produce visible colored images.
Heretofore, microcapsules have been produced by a variety of physical and chemical methods. For example, one method involves the phenomenon of "complex coacervation", wherein two oppositely charged colloids, such as gelatin and gum arabic, are utilized under carefully controlled conditions of pH, temperature, and concentration to form a liquid wall around dispersed oil droplets containing a chromogen, which liquid wall is subsequently hardened by further chemical action. A unique feature of coacervate capsular systems is that the final microcapsules are produced in a "cluster" or "grape-like" configuration.
Several other, non-coacervate microencapsulation systems have been described in U.S. Pat. Nos. 3,779,941, 3,875,074, and 3,886,084. Most of these systems involve the formation of microcapsules by interfacial cross-linking methods, whereby a cross-linking agent is incorporated and allowed to react with a polymeric material having cross-linkable sites to form the capsule wall. While these non-coacervate systems produce microcapsules which are discrete and non-agglomerated, they do possess an inherent weakness in that the walls of the capsules are thin and exhibit a high degree of porosity. Therefore, if the "core" or encapsulated material is highly volatile or easily sublimable, it can not be retained within the capsule wall for prolonged periods of time.